Method for printing by using a multilevel character generator and printing device

ABSTRACT

A method in which a multilevel character generator ( 22 ) is adjusted using a single correction factor (KF) during an automatic compensation operation such that high-quality printed images can be obtained also in the instance of modified printing conditions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a method for printing with amulti-level character generator. The invention is also directed to aprinter device that is explained in greater detail below.

2. Description of the Related Art

A method is known wherein an optical character generator illuminates aphotoconductor with at least one light source. Light encoding data thatrespectively contain one of at least three different light encodingvalues are generated from print data of a print image. The lightencoding values are respectively allocated to a different illuminationenergy value with which the character generator illuminates thephotoconductor.

In contrast to traditional bi-level character generators, charactergenerators driven with more than two light encoding values are referredto as multi-level character generators. Although there are more than twolight encoding values given multi-level character generators, there areultimately only printed or non-printed surfaces. Compared to bi-levelcharacter generators, however, multi-level character generators offerthe possibility of designationally defining the size of the chargeregions and, thus, of the pixels (picture elements) in order to producethe impression of different gray scale values for someone who views thedeveloped charge image. Such a multi-level character generator isdisclosed by U.S. Pat. No. 5,767,888.

What is disadvantageous about the known printing with multi-levelcharacter generators is that the quality is reduced given changingprinting conditions. For example, these printing conditions include theincreasing age of the photoconductor and changes in the quality of thetoner or, respectively, developer. Methods with which print imageshaving good quality can be printed even given modified printingconditions are employed for printers with bi-level character generators,see, for example, the balancing method set forth in PublishedInternational Patent Application WO 97/37285.

Given LED character generators, moreover, auxiliary parameters withwhose assistance manufacture-conditioned deviations in the lightemission of the individual LEDs are compensated is usually prescribedfor each LED. All LEDs therefore emit the same illumination energy giventhe same light encoding value. Such a balancing is disclosed, forexample, in European Patent EP 0 275 254 B1.

German Published Application DE 37 27 808 A1 discloses an imagerecording device wherein topical deviations of the sensitivity of aphotoconductor are determined by comparison to a reference potential andare subsequently corrected. The correction ensues topically dependentwith different correction quantities when printing various regions ofthe print image.

U.S. Pat. No. 4,709,250 discloses a method for printing with theassistance of a character generator. The potential V is acquired at agiven balance illumination energy, whereby the balance illuminationenergy derives from the current intensity flowing through the laser orLED together with the illumination time on the photoconductor. Theacquired potential V is compared to a reference potential. Theillumination energy values are modified on the basis of this comparison.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for theoperation of an electrophotographic printer or copier device withmulti-level character generation that makes it possible to generateprint images having high print quality even given changing printingconditions. Further, a printer device with which the method can beimplemented is provided.

The objects of the invention are achieved by a method for printing witha character generator,whereby an optical character generator illuminatesa photoconductor with at least one light source, light encoding data aregenerated from print data of a print image, the light encoding datarespectively contain one of at least three different light encodingvalues that are allocated to different illumination energy values, abalance potential to be set on the photoconductor is prescribed for abalance event, the illumination energy producing the prescribed balancepotential on the photoconductor is acquired as balance illuminationenergy, and whereby the illumination energy values to be generated bythe character generator given the various light encoding values aremodified in the same relationship dependent on the acquired balanceillumination energy, whereby the degree of the modification isdetermined by the deviation of the balance illumination energy from areference illumination energy that generates the balance potential givenemployment of a reference photoconductor with a predetermined dischargecharacteristic.

In a preferred embodiment, a correction factor is determined that is acriterion for the deviation of the current characteristic of thephotoconductor from the predetermined discharge characteristic; and theillumination energy values are modified with the assistance of thecorrection factor. Specifically, the amount of the balance potential or,respectively, of the acquired balance potential deviates by less than100 V from the amount of the discharge potential of the photoconductor.The amount of the predetermined balance potential or, respectively, ofthe acquired balance potential of a preferred embodiment lies no morethan 50 Volts from the average of the amount of the discharge potentialof the photoconductor and the amount of the discharge potential. Thecorrection factor for setting the character generator is determinedpreferably by division from the acquired balance illumination energy andthe reference illumination energy or, respectively, from the acquiredbalance potential and the reference potential. The acquired balanceillumination energy or, respectively, the acquired balance potential maybe employed as a correction factor for the setting of the charactergenerator. A current for the drive of the light sources or,respectively, of the light sources of the character generator or theiron duration is modified dependent on the correction factor. Anapproximation for the characteristic of the photoconductor is preferablyemployed for determining the deviation or for determining the correctionfactor. The following approximation is employed:

VD(K,T,H)=(VC-VLIM)·exp(−K·T·H)+VLIM,  (1)

whereby

VC is the charge potential of the photoconductor in volts,

VD is the discharge potential of the photoconductor in volts,

VLIM is the lowest obtainable discharge potential in volts,

H is the illumination energy in μWs/cm2,

T is the currently acquired temperature of the photoconductor in ° C.,

K is the photoconductor class in cm2/(μWs° C.), and

exp is the exponential function.

In one aspect of the invention, the balancing event is automaticallyimplemented, preferably after a printer or, respectively, copier deviceis turned on and/or after longer printing pauses and/or after longerprinter operation and/or on demand of an operator. A further featureprovides that the discharge characteristic of the referencephotoconductor is permanently prescribed, particularly independently ofmodifications due to aging or modified ambient conditions in theprinting process.

The present invention also provides a printer or copier device,particularly for the implementation of the preceding methods, includinga print data unit that generates light encoding data having respectivelyone of at least three different light encoding values from print data ofa print image, a multi-level character generator that is driven with thelight encoding data and emits a predetermined illumination energydependent on the respective light encoding value, and a photoconductorthat is discharged by the illumination energy output by the charactergenerator, whereby a balancing device is provided that, in an automaticbalance event, modifies the illumination energies emitted by thecharacter generator at different light encoding values in the samerelationship, means are provided for the balance event in order toprescribe a balance potential to be set on the photoconductor, means areprovided that acquire the illumination energy producing thepredetermined balance potential on the photoconductor as balanceillumination energy, the illumination energy values to be generated bythe same generator given the various light encoding values are modifiedby the balance device in the same relationship dependent on the acquiredbalance illumination energy, and whereby the degree of the modificationis determined by the deviation of the balance illumination energy from areference illumination energy that generates the balance potential givenemployment of a reference photoconductor with a predetermined dischargecharacteristic.

The invention is based on the perception that, due to the incompletelinear characteristic of the photoconductor, the illumination energy isactually to be individually balanced for each light encoding value inthe balancing of a multi-level character generator. However, there isalso an adequately exact balancing for many applications when a commonbalancing procedure is implemented for the illumination energies of thedifferent light encoding values. In the inventive method, theillumination energies generated given the different light encodingvalues are therefore modified in the same relationship. Most of thelight encoding values have illumination energies that lie on a linearregion of the photoconductor characteristic, so that the illuminationenergies for the different light encoding values can be set withadequate precision as a result of the correction ensuing in the samerelationship.

Given the method of the invention, a balancing potential to be set onthe photoconductor is prescribed for the balancing event. Subsequently,the illumination energy that produces the predetermined balancingpotential on the photoconductor is acquired and a balanced illuminationenergy determined. Dependent on the acquired balance illuminationenergy, the illumination energy values to be generated by the charactergenerator given the various light encoding values are modified in thesame relationship.

The extent of the modification is determined by the deviation of thebalance illumination energy from a reference illumination energy. Thereference illumination energy is the illumination energy that generatesthe balance potential given employment of a reference photoconductorwith a predetermined discharge characteristic. The dischargecharacteristic indicates the relationship of illumination and potential.The reference photoconductor, for example, is a photoconductor in a newprinter device that is operated at 20° C. room temperature.

In the inventive method, the illumination energy values generated by thecharacter generator at the different light encoding values are modifiedin the same relationship relative to one another and are thus reset. Theinventive method is simple because all illumination energies aremodified in the same relationship. Nonetheless, print images having aprint quality that is sufficient for many purposes derive. The higherprint quality that can be achieved given a multi-level charactergenerator compared to a bi-level character generator is also assuredunmodified by the inventive method when the properties of thephotoconductor and/or of the electrographic development system change.

In one development, a correction factor is determined that is acriterion for the deviation of the current characteristic of thephotoconductor from the predetermined discharge characteristic. Theillumination energies are then modified in a simple way with theassistance of the correction factor. This ensues in that, for example,the illumination energy value predetermined for each light encodingvalue is multiplied by the correction factor.

In another development of the inventive method, the amount of thepredetermined or, respectively, acquired balance potential deviates byless than 100 V from the amount of the discharge potential of thephotoconductor. For example, the amount of the balance potential liesonly slightly above what is the lowest discharge potential in terms ofamount. In other words, the discharge potential is the potential that isestablished on the photoconductor when the photoconductor is completelydischarged. Given certain photoconductors, the discharge potential isrelatively independent of the printing conditions. For example, ithardly changes with increasing age of the photoconductor or withdifferent temperature. The discharge potential is therefore especiallywell-suited for identifying the balance illumination energy.

In an alternative development of the inventive method, the amount of thepredetermined balance potential lies roughly at the average from theamount of the charge potential of the photoconductor and the amount ofthe discharge potential and thus lies in the linear region of thephotoconductor characteristic. In particular, only this linear part ofthe photoconductor characteristic is employed in the printing givenhigh-speed printing.

In a next development, the correction factor is determined by divisionof the acquired balance illumination energy by the referenceillumination energy. Alternatively, however, the acquired balanceillumination energy or, respectively, the acquired balance potential canalso be immediately used as a correction factor for the charactergenerator.

In one development, an approximation for the characteristic of thephotoconductor is employed for the determination of the illuminationenergies to be emitted by the character generator. The involvement ofthe characteristic makes it possible to implement only a singlemeasurement at the photoconductor. For a predetermined illuminationenergy, for example, the potential occurring given an illumination ofthe photoconductor with this illumination energy is measured.Subsequently, the balance illumination energy belonging to thepredetermined balance potential or, respectively, the balance potentialbelonging to the predetermined balance illumination energy is determinedfrom the characteristic, which changes with the printing conditions.

The balancing event is automatically implemented. This preferably ensuesafter a printer or, respectively, copier device is turned on, afterlonger printing pauses, after a longer printing operation and/or ondemand of an operator. These measures assure that modifications of thephotoconductor due to aging or due to altered ambient conditions aretaken into consideration in the printing process.

In one development, the illumination energy values are modified in thesame relationship both with respect to one another as well as forpicture elements. The correction thus identically influences all pictureelements.

The invention is also directed to a printer or, respectively, copierdevice that, in particular, is employed for the implementation of theinventive methods or, respectively, their developments. Theaforementioned technical effects thus also apply to the inventiveprinter or, respectively, copied device.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained below on the basisof the attached drawings.

FIG. 1 is a schematic illustration of a printing event in block diagram.

FIG. 2 is a graph showing a potential/illumination energy diagram.

FIG. 3 is a flowchart with method steps of a balancing event.

FIG. 4 is a list of equations for the approximation of a photoconductorcharacteristic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a schematic illustration of a printing event as well as theinformation flow when printing in an electrophotographic printer device10. Print data 14 are input in the printer 10 via an electronicinterface 12. The print data 14 define a print image, for exampleaccording to the known postscript format. A print data unit 16 containsa microprocessor MP that processes a conversion program stored in amemory 18. The print data unit 16 generates light encoding data 20 forthe individual LEDs (light-emitting diode) of a character generator 22from the print data 14. The light encoding data 20 are respectivelystored in two bits of a data word. There are thus four light encodingvalues 0, 1, 2 and 3. No illumination is carried out given the lightencoding value 0. Given the light encoding value 1, an illuminationenergy H1 is emitted by the appertaining LED of the character generator22 when the printer device 10 prints under predetermined printingconditions in a reference printing event characterized by these printingconditions. The light encoding value 2 or, respectively, 3 leads toillumination energies H2 or, respectively, H3 in the reference printingevent. The illumination energies H1, H2 and H3 have ascending values inthis sequence and are shown in FIG. 2.

The light encoding data 20 are processed in a conversion and correctionunit 24 that is constructed essentially like the assembly shown in FIG.12 of U.S. Pat. No. 5,767,888. Before the beginning of the printingevent, a correction factor KF is set at the conversion and correctionunit 24 in that the correction factor KF is respectively multiplied bythe auxiliary parameters D1 through D64 shown in FIG. 12 of U.S. Pat.No. 5,767,888 that serve for the compensation of themanufacture-conditioned brightness fluctuations of the LEDs. Theproducts that thereby derive are employed instead of the values D1through D64. The method steps to be implemented for the determination ofthe correction factor KF are explained in greater detail below on thebasis of FIG. 2.

A light signal 26 is determined in the conversion and correction unit 24dependent on the light encoding value of the respectively processedlight encoding datum and dependent on the correction factor KF. Giventhe light encoding value 0, a light encoding signal 26 is generated thatleads to no illumination by the appertaining LED. Given the lightencoding value 1, a light encoding signal 26 is generated that—takingthe correction factor KF into consideration—effects an illumination witha corrected illumination energy H1 a. Given the light encoding value 2or, respectively, 3, a light encoding signal 26 is generated that—takingthe correction factor KF into consideration—leads to an illuminationwith a corrected illumination energy H2 a or, respectively, H3 a.

The light encoding signals 26 are employed for the drive of thecharacter generator 22. The character generator 22 contains a drivecircuit for the LEDs of an LED line (not shown). Such a drive circuit isdisclosed by U.S. Pat. No. 5,767,888. The LEDs of the LED line have aspacing of approximately 42 μm from one another. What are referred to asmacro-cells are respectively generated by three LEDs arrangedsequentially when illuminating three successive lines. Light energydistributions 28 emitted by the LEDs of a macro-cell partiallysuperimpose on one another. Given a suitable selection of the lightencoding values, light distribution increases that lead to adistribution 32 of potential on a photoconductor 30 similar to therespective light distribution arise within a macro-cell. What isachieved by defining a threshold of potential in a developing unit 34 isthat toner regions 36 whose diameters are dependent on the fashioning ofthe respective potential distribution form in the macro-cells. Ahalf-tone image is thus generated in a simple way by employing the lightencoding values. This procedure, including the character generator 22employed therein as well as the term “macro-cell”, is set forth indetail in U.S. Pat. No. 5,767,888, which is a constituent part of thedisclosure of the present application and is incorporated by referenceherein.

FIG. 2 shows a potential/illumination energy diagram on whose abscissaaxis 50 the illumination energy is shown in μWs/cm² and on whoseordinate axis 52 the photoconductor potential is shown in volts. Beforethe beginning of the illumination, the photoconductor is charged to acharge potential VC of 500 V. A reference characteristic KLB shows therelationship of potential on the photoconductor and illumination energyfor a reference photoconductor that is employed in the referenceprinting event. A characteristic KL1 of a photoconductor employed forprinting at the moment deviates from the reference characteristic KLB.The deviations of the characteristics KLB and KL1 are to be attributed,for example, to the temperature or to the age of the photoconductor.Deviations of the characteristics KLB and KL1, however, also arise givena change of the photoconductor or, respectively, when comparing thephotoconductors of two different printers. In this case,manufacture-conditioned fluctuations as well as the quality of thephotoconductors have an additional influence on the deviation of thecharacteristics KLB and KL1.

A further characteristic KL2 shows the dependency of the potential on athird photoconductor on the illumination energy. Qualitatively, thecharacteristics KLB, KL1 and KL2 have a similar curve, so that only thecurve of the characteristic KLB shall be explained below. Withincreasing illumination energy, the values of potential on thephotoconductor drop according to a descending exponential function untila lowest obtainable discharge potential VLIM is finally reached,illustrated with a broken line 54.

The printing event leads to print images with high print quality whenthe potentials that are generated upon occurrence of the various lightencoding values 0, 1, 2 or, respectively, 3 have an approximatelyuniform spacing from one another and are distributed over the entiredischarge region that is available. However, only potentials in theupper region of the discharge curve are taken into consideration in thefollowing explanation in order to simplify the explanations. Noillumination is carried out given the light encoding value 0, so thatthe charge potential VC is retained. The potential V1=450 V, forexample, should be generated given the light encoding value 1. Apotential V2=400 V or, respectively, V3=350 V should be generated giventhe light encoding value 2 or, respectively, 3. On the referencecharacteristic KLB, the illumination energy H1 belongs to the potentialV1, the illumination energy H2 belongs to the potential V2 and theillumination energy H3 belongs to the potential V2.

FIG. 3 shows a flowchart for a first exemplary embodiment of thebalancing event. FIG. 2 is also referenced in the explanation of FIG. 3.The balancing event begins after the activation of the printer in a step100. For determining the correction factor KF, the current illuminationscope HL of the photoconductor is acquired in a step 102 in that thephotoconductor is initially charged to the charge potential VC.Subsequently, the illumination energy is raised step-by-step until abalance potential VA to which the following applies is acquired:

VA=|VC|−0.95|VC−VLIM|  (1)

The illumination scope H1 is thus defined as the illumination energy HLat which the photoconductor is 95% discharged. The illumination scope HLof the photoconductor with the characteristic KLB is a referenceillumination scope HLB. An illumination scope HL1 belongs to thecharacteristic KL1.

In step 104, the correction factor KF is calculated according to theequation: $\begin{matrix}{{KF} = \frac{HL1}{HLB}} & {(2).}\end{matrix}$

The correction factor amounts to approximately 0.6 for thecharacteristic KL1. This means that the illumination energies H1, H2 or,respectively, H3 for the light encoding values 1, 2 or, respectively, 3are respectively multiplied by the correction factor KF=0.6. To thatend, as already explained above, the auxiliary parameters for thebalancing of the light-emitting diodes to the same brightness aremultiplied by the correction factor KF in the conversion and correctionunit 24 and are stored, see step 106. When printing in step 108, lightenergies H1 a, H2 a or, respectively, H3 a arise that are employedinstead of the illumination energies H1, H2 or, respectively, H3. Theillumination energies H1 a, H2 a and H3 a also lead to the potentialsV1, V2 or, respectively, V3 given employment of a photoconductor withthe characteristic KL1 and, thus, to a high-quality print image. Themethod is ended in a step 110. The steps 100 through 110 are run uponemployment of a microprocessor.

An illumination scope HL2 that is greater than the referenceillumination scope HLB would be correspondingly determined given aphotoconductor with the characteristic KL2. The correction factor KF istherefore greater than 1, for example 1.4, so that the illuminationenergy H1 is increased to a value H1 b (not shown). The illuminationenergy H2 is likewise increased to an illumination energy H2 b or,respectively, the illumination energy H3 is increased to an illuminationenergy H3 b.

Despite the non-linear characteristics KL1, KLB and KL2, the correctionwith only one correction factor KF leads to adequately exact settings ofthe illumination energies allocated to the light encoding values 1, 2and 3.

In a second exemplary embodiment, an illumination energy HE isdetermined that discharges the photoconductor to a balance potential VEAthat lies about in the middle between the charge potential VC and thedischarge potential VLIM. After being charged to the charge potentialVC, the photoconductor is more and more intensely illuminated in stepsuntil the balance potential VEA is acquired. The value of theillumination energy HE is then employed as a correction factor KF. Theillumination energy HE1 to be set derives for the characteristic KL1,this lying by about the factor 0.6 below an illumination energy HEwhereat the balance potential VEA occurs on the reference photoconductorwith the characteristic KLB. An illumination energy HE2 to be setderives for the characteristic KL2, this lying above the illuminationenergy HE by approximately the factor 1.4.

It is assured in a calibration event at the factory that, given acorrection factor KF=HE, the character generator is driven with acurrent that leads thereto that the character generator outputs anillumination energy that results in the potential VEA at the referencephotoconductor with the characteristic KLB. The value HE1 is employed asa correction factor given the characteristic KL1 and the value HE2 isemployed given the characteristic KL2. The illumination energies for thelight encoding values 0 through 3 are co-set in this balancing eventbecause the setting of the current for the drive of the charactergenerator 22 influences all illumination energies in the samerelationship.

FIG. 4 shows equations (1), (2) and (3) that are employed in a thirdexemplary embodiment in the determination of the correction factor KF.The equations (1), (2) and (3) are explained below, likewise withreference to FIG. 2. Equation ( 1 ) reads:

VD(K,T,H)=(VC−VLIM)·exp(−K·T·H)+VLIM,  (1)

whereby

VC is the charge potential of the photoconductor in volts,

VD is the discharge potential of the photoconductor in volts,

VLIM is the lowest obtainable discharge potential in volts,

H is the illumination energy in μWs/cm²,

T is the currently acquired temperature of the photoconductor in ° C.,

K is the photoconductor class in cm²/(μWs° C.), and

exp is the exponential function.

Equation (1) is an approximation for the respective characteristic ofthe photoconductor. The characteristics KLB, KL1 and KL2 in FIG. 2differ from one another on the basis of the photoconductor class K.Equation (2) arises by reformulating Equation (1) according to thephotoconductor class K: $\begin{matrix}{{{K\left( {{VD},T,H} \right)} = {\frac{1}{T \cdot H}\quad \ln \quad \left( \frac{{VC} - {VLIM}}{{VD} - {VLIM}} \right)}},} & (2)\end{matrix}$

whereby

ln is the logarithm function.

When a standard illumination energy HS is prescribed for theillumination energy H and, following illumination of the photoconductorwith this illumination energy HS, the arising discharge potential VD aswell as the temperature T of the photoconductor are acquired, then allquantities on the right side of Equation (2) are known and thephotoconductor class K can be calculated. Alternatively, tables can beemployed wherein photoconductor classes K calculated once for specificvalues of VD, T and H are stored.

When Equation (1) is reformulated according to the illumination energyH, then Equation (3) derives: $\begin{matrix}{{{H\left( {{VD},K,T} \right)} = {\frac{1}{T \cdot K}\quad \ln \quad \left( \frac{{VC} - {VLIM}}{{VD} - {VLIM}} \right)}},} & {(3).}\end{matrix}$

After the photoconductor class K has been determined, the potentials VEAis inserted for the discharge potential VD. Prepared tables can therebybe employed in order to implement the determination of the correctedillumination energies HE1 or, respectively, HE2 fast. Given acorresponding calibration of the character generator, for example, thevalue of the illumination energy HE1 is employed as value of thecorrection factor.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventor to embody withinthe patent warranted hereon all changes and modifications as reasonablyand properly come within the scope of their contribution to the art.

What is claimed is:
 1. A method for printing with a character generator,comprising the steps of: illuminating a photoconductor with at least onelight source of an optical character generator; generating lightencoding data from print data of a print image; allocating at leastthree different light encoding values contained in said light encodingdata to different illumination energy values; prescribing a balancepotential to be set on the photoconductor for a balance event; acquiringan illumination energy producing the prescribed balance potential on thephotoconductor as a balance illumination energy; modifying theillumination energy values to be generated by the optical charactergenerator given the different light encoding values in a samerelationship dependent on the acquired balance illumination energy; anddetermining a degree of modification by a deviation of the balanceillumination energy from a reference illumination energy that generatesthe balance potential given employment of a reference photoconductorwith a predetermined discharge characteristic.
 2. A method as claimed inclaim 1, further comprising the steps of: determining a correctionfactor that is a criterion for deviation of the current characteristicof the photoconductor from the predetermined discharge characteristic;and modifying the illumination energy values with assistance of thecorrection factor.
 3. A method as claimed in claim 2, wherein said stepof determining the correction factor sets the correction factor for thecharacter generator by one of division from the acquired balanceillumination energy and the reference illumination energy and from theacquired balance potential and the reference potential.
 4. A method asclaimed in claim 2, further comprising the step of: utilizing one of theacquired balance illumination energy and the acquired balance potentialas a correction factor for setting of the character generator.
 5. Amethod as claimed in claim 2, further comprising the step of: modifyinga current for driving the at least one light source of the charactergenerator depending on the correction factor.
 6. A method as claimed inclaim 2, further comprising the step of: modifying an on time durationof the at least one light source dependent on the correction factor. 7.A method as claimed in claim 1, wherein an amount of one of the balancepotential and an acquired balance potential deviates by less than 100volts from an amount of the discharge potential of the photoconductor.8. A method as claimed in claim 1, wherein the amount of one of thebalance potential and the acquired balance potential lies no more than50 Volts from an average of an amount of a discharge potential of thephotoconductor and an amount of the discharge potential.
 9. A method asclaimed in claim 1, further comprising the step of: utilizing anapproximation for a characteristic of the photoconductor for determiningthe deviation.
 10. A method as claimed in claim 9, wherein the followingapproximation is employed: VD(K,T,H)=(VC−VLIM)·exp(−K·T·H)+VLIM,  (1)whereby VC is the charge potential of the photoconductor in volts, VD isthe discharge potential of the photoconductor in volts, VLIM is thelowest obtainable discharge potential in volts, H is the illuminationenergy in μWs/cm2, T is the currently acquired temperature of thephotoconductor in ° C., K is the photoconductor class in cm2/(μWs° C.),and exp is the exponential function.
 11. A method as claimed in claim 1,further comprising the step of: utilizing an approximation for acharacteristic of the photoconductor for determining the correctionfactor.
 12. A method as claimed in claim 1, wherein the balancing eventis automatically implemented.
 13. A method as claimed in claim 12,wherein said automatic implementation is one of after a printer orcopier device is turned on and after longer printing pauses and afterlonger printer operation and on demand of an operator.
 14. A method asclaimed in claim 1, wherein the discharge characteristic of thereference photoconductor is permanently prescribed.
 15. A method asclaimed in claim 14, wherein said permanent prescription is independentof modifications due to aging or modified ambient conditions in theprinting process.
 16. A printer or copier device, comprising: a printdata unit that generates light encoding data having one of at leastthree different light encoding values from print data of a print image;a multi-level character generator that is driven with the light encodingdata and emits a predetermined illumination energy dependent on therespective light encoding value; a photoconductor that is discharged bythe illumination energy output by the multi-level character generator; abalancing device that in an automatic balance event modifies theillumination energies emitted by the character generator at differentlight encoding values in a same relationship; a controller for a balanceevent to prescribe a balance potential to be set on the photoconductor;a sensor to acquire the illumination energy producing a predeterminedbalance potential on the photoconductor as a balance illuminationenergy; said character generator modifying the illumination energyvalues generated by the same generator given the various light encodingvalues are modified by the balance device in the same relationshipdependent on the acquired balance illumination energy, and the degree ofthe modification is determined by the deviation of the balanceillumination energy from a reference illumination energy that generatesthe balance potential given employment of a reference photoconductorwith a predetermined discharge characteristic.